Propagation of radio frequencies is particularly desirable under some circumstances. The classic case of desirability is where a radio broadcast facility intends to transmit a radio wave into the atmosphere. In such a case, an antenna having a particular length and structure compatible with the wavelength and frequency of the waves to be transmitted is designed and constructed. The antenna is constructed specifically in order to maximize propagation of the wave.
While in the broadcasting scenario maximization of transmission is desirable, it is, at best, undesirable, and at worst, totally unacceptable, in other scenarios. The advent of the computer, including personal computers, and other equipments such as dedicated wordprocessors and electronic telephone systems has given light to circumstances where transmission of radio frequencies from transmission lines is undesirable. Power cables and transmission lines radiate radio frequencies for some distance into spaces which electronic equipments, of which the power cables and transmission lines are a part, occupy. Under some circumstances, there is no great problem because the frequencies are not radiated sufficient distances to create noise problems for other equipments. Typically, equipments in adjacent spaces are positioned so that noise problems are not made apparent.
In other circumstances, significant noise detriment can occur. This is often significantly true when an equipment generating RF noise has a power cable which plugs into a conventional wall outlet at the same location at which a wall outlet in an adjacent space is positioned. There can be a direct transmission of the noise from a generating equipment, along its power cable to its wall outlet, and directly through to the other side of the wall and into a power cable of an equipment in the adjacent space. When this occurs, a damaging effect results upon the operation of the equipment receiving the RF noise. For example, if the equipment is a computer, the effects upon operation of the computer may immediately become apparent in viewing the CRT.
There are two kinds of noise that can be generated. The first is conducted noise. Such noise is transmitted axially along the length of the transmission line. The second type is radiated noise; radiated noise emanates radially outwardly from the transmission line as would radio transmissions from, for example, a commercial radio station antenna.
While conducted noise can do much damage, it is often not as major a problem as is radiated noise. Consequently, those in the industry are, typically, not as concerned about conducted noise as they are about radiated noise. The problem can become critical, however, because conducted noise can become radiated noise if allowed to traverse cables or transmission lines.
Because of problems that have developed in the industry over the past decade or so, national agencies in various countries have implemented regulations and requirements which must be met by electronic equipments placed on the market.
Various attempts have been made to solve the problems created by conducted and radiated noise. One attempted solution has been to try to filter each individual pin of, for example, a D subminiature connector which functions to connect a data transmission line. The attempted filtering of each individual pin is, however, complex and, consequently, expensive. Even when filtering of this nature is able to be accomplished, the result is less than optimum, since such filtering does not effect filtering of noise on the cable shield.
Another attempted solution has been to place an annular ferrite sleeve over the transmission line or cable itself. Typically, however, such sleeves are brittle. Consequently, in view of their generally exposed disposition, they can become cracked, broken, and, as a result, unable to perform their intended function. Further, no magnetic shield is provided over the sleeve to eliminate magnetic interference to nearby components such as cathode ray tubes.
Additionally, when such a solution is sought, aesthetic problems result. An obtrusive appendage makes a product employing such a sleeve unsightly.
A further problem that is encountered when seeking to implement a ferrite sleeve solution is that of location. Sleeves that are affixed to a transmission line locate the sleeve some distance from the interconnecting pins thus leaving exposed or unfiltered lengths of transmission line causing radiation. Another problem encountered with the prior art is that the sleeve is often not captive but can move down the transmission line rendering it ineffective.
It is to these problems and desirable features dictated by the prior art that the present invention is directed. It is both an apparatus and method for reducing radio frequency noise emanating from a transmission line, without incurring the drawbacks of the prior art.